The present disclosure relates to an engine supercharger to be mounted on vehicles such as motor vehicles.
To increase its output, a vehicle engine is sometimes provided with a turbocharger for supercharging the engine with pressurized air by utilizing the energy of its exhaust gas.
A turbocharger generally includes a turbine and a compressor as its major components. The turbocharger turns the turbine with an exhaust gas released from the engine to generate power that drives the compressor, thereby increasing the manifold air pressure.
The engine speed of a motor vehicle varies significantly according to its traveling condition. Thus, a turbocharger mounted on a motor vehicle is required to exhibit stabilized supercharging performance irrespective of the amount of exhaust gas released, which varies widely from a low-speed operating region where the amount of the exhaust gas released is relatively small through a high-speed operating region where the amount of the exhaust gas released is relatively large.
Japanese Unexamined Patent Publication No. 2008-31942 discloses a technique for improving the performance of such a turbocharger by classifying a group of exhaust passages, through which an exhaust gas is introduced from a plurality of cylinders of an engine into a turbine, into first and second exhaust systems 161 and 162.
Specifically, according to the technique disclosed in Japanese Unexamined Patent Publication No. 2008-31942, the two exhaust systems 161 and 162 have substantially equal flow cross sections, but the first exhaust system 161 is designed to have a shorter passage length and a smaller capacity than the second exhaust system 162. Also, the first exhaust system 161 is introduced into an outer peripheral portion of the turbine 21 and the second exhaust system 162 is introduced into an inner peripheral portion of the turbine 21.
This allows an exhaust gas with a relatively high flow velocity to be introduced into the turbine through the outer peripheral portion, thereby enabling the turbocharger to achieve high supercharging performance in the low-speed operating region.
However, the turbocharger disclosed in Japanese Unexamined Patent Publication No. 2008-31942 still has room for improvement regarding how to use the exhaust gas in an operating region where the amount of the exhaust gas released is relatively small. The reason is that the turbocharger allows the exhaust gas to flow continuously through both of the two exhaust systems.
Meanwhile, Japanese Unexamined Patent Publication No. 2009-144642 discloses an engine system including a variable exhaust valve 30, which varies the cross-sectional area of the opening (i.e., flow cross section) of its exhaust passage according to the engine's operating region, such that the opening area of the exhaust passage is narrowed while the engine is operating in a region where the amount of exhaust gas released is small.
Specifically, according to Japanese Unexamined Patent Publication No. 2009-144642, a duct 51b for introducing the exhaust gas into a turbine is split into an inner portion and an outer portion by a rectifier 56. The turbocharger disclosed in Japanese Unexamined Patent Publication No. 2009-144642 is configured such that when the opening area is decreased by the variable exhaust valve 30, the outer passage is closed and the mainstream exhaust gas is guided by the rectifier 56 into the inner portion of the turbine.
The rectifier 56 enters a turbine chamber 51c while bending inward. More specifically, a terminal portion 56a of the rectifier 56 is extended to the point of facing a tongue portion 51d of a turbine casing 51a (i.e., a portion where the turbine chamber's scroll starts to swirl and which is located close to a turbine wheel) with the turbine wheel interposed between them. That is to say, the terminal portion 56a is arranged close to the turbine wheel.
This allows the turbine wheel to receive great couple of force Fc from the exhaust gas at the terminal portion 56a of the rectifier 56 and the tongue portion 51d, thus applying a great moment to the turbine wheel while decreasing the bending moment acting on a shaft 53 of the turbine wheel.
However, if the rectifier is allowed to extend deep into the turbine chamber as disclosed in Japanese Unexamined Patent Publication No. 2009-144642, then the exhaust gas flowing into the turbine chamber through the inner passage is rather separated from the exhaust gas flowing into the turbine chamber through the outer passage. That is why if the exhaust gas is introduced into the turbine through not only the inner passage but also the outer passage, then the exhaust gas flows coming from the respective cylinders will apply their pressure to the turbine wheel at noticeably different timings, thus generating significant energy loss.
In view of the foregoing background, it is therefore an object of the present disclosure to provide an engine supercharger allowing an exhaust gas to act on a turbine highly efficiently over a wide operating range.